In recent years, composite materials comprising highly filled polymers have become commonly used for dental restorations. Current composite materials contain crosslinking acrylates or methacrylates, inorganic fillers such as glass or quartz, and a photoinitiator system suitable for curing by visible light. Typical methacrylate materials include 2,2′-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (“Bis-GMA”); ethoxylated Bisphenol A dimethacrylate (“EBPDMA”); 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane (“UDMA”); dodecanediol dimethacrylate (“D3MA”); and triethyleneglycol dimethacrylate (“TEGDMA”). The structural formulae for these are shown below.

Dental composite materials offer a distinct cosmetic advantage over traditional metal amalgam. However, they do not offer the longevity of amalgam in dental fillings. The primary reason for failure is excessive shrinkage during photopolymerization in the tooth cavity, which can cause leakage and bacterial reentry. Another reason is they have inadequate strength and toughness, as reflected in the measured properties of flexural strength and fracture toughness. Hence, there is still a need for new monomers and new monomer combinations which, when polymerized, impart high fracture toughness and flexural strength in the resulting composite. It is also highly desirable to have low shrinkage stress on polymerization.
WO 01/95862 A1 summarizes several teachings from the prior art. The first teaching is that, while it is known that increasing the molecular weight of the monomers used in making a composite decreases the polymerization shrinkage of the composite, using higher molecular weight monomers undesirably increases viscosity. The prior art has taught the use of low viscosity reactive diluents such as TEGDMA when the desired monomers are too viscous to make a good composite. However these reactive diluents typically have very high polymerization shrinkages and they compromise the properties of the final composite.
One of the more common commercially used monomers is Bis-GMA. However, it is highly viscous at room temperature and difficult to work with. It is therefore diluted with a second, lower viscosity polymerizable component (“fluidizer”), a methacrylate monomer, such as TEGDMA, tetraethylene glycol dimethacrylate, or dodecanediol dimethacrylate. However, while providing low viscosity, lower viscosity components (generally low molecular weight monomers) can contribute to increased shrinkage. Increasingly, Bis-GMA and TEGDMA have been combined with UDMA and EBPDMA, but shrinkage remains high enough that improvement is desirable.
Urethane (meth)acrylates are common constituents of curable adhesives, coatings, printing inks and dental materials. One monomer employed frequently in the dental field is 7,7,9-trimethyl-4,13-dioxo-3,14-dioxa-5,12-diazahexadecan-1,16-diyldimethacrylate (UDMA). UDMA is prepared by the reaction of one mole of 2,2,4-trimethylhexamethylene diisocyanate with two moles of 2-hydroxymethyl methacrylate (HEMA) and is a viscous liquid at room temperature. UDMA is a relatively small molecule, with a mass per double bond near 235 Daltons. When employed as a major component in light-curable dental composite formulations, UDMA can lead to unacceptably high levels of polymerization shrinkage that ultimately compromise the durability and performance of the cured restorative.
In addition to UDMA, other urethane based dimethacrylates have been disclosed. For example, M. G. Buonocore and C. A. Casciani (New York State Dental Journal 1969, 35, 135) describe the reaction products of two moles of HEMA with one mole each of 2,4-toluoylene diisocyanate, naphthylene diisocyanate, hydrogenated diphenylmethane diisocyanate or hexamethylene diisocyanate. However, these products are crystalline compounds that require the use of liquid comonomers for the formulation of suitable dental composites.
U.S. Pat. No. 6,653,375 B2 discloses a series of urethane dimethacrylates for use in dental composites that are based on 1,3-bis(1-isocyanato-1-methylethyl)benzene (“TMXDI”). The urethanes described in this patent have (meth)acrylate arms one to eight carbon atoms long, optionally interrupted by oxygen atoms.
U.S. Pat. No. 4,243,578 relates to a series of urethane dimethacrylates prepared from the reaction of diisocyanate starting materials with C1-8-hydroxyalkyl methacrylates and their use in dental filling materials.
U.S. Pat. No. 4,400,159 discloses the reaction of diisocyanates with 3-methylacroyl-2-hydroxypropyl esters to give highly branched urethane dimethacrylate analogues with short connecting arms, typically two carbon atoms and two oxygen atoms long, for use in dental restorative formulations.
U.S. Pat. No. 4,110,184 discloses the reaction of urethane containing pre-polymers with hydroxyalkyl (meth)acrylates and the use of these materials for the formulation of dental filling compositions. Short arms, two carbon atoms long, connecting the (meth)acrylate group and the urethane core are preferred.
U.S. Pat. No. 3,931,678 describes urethane (meth)acrylate monomers formed from the reaction of an organic polyisocyanate with a polymerizable (meth)acrylate ester containing reactive hydroxyl or amine groups, and the use of such materials in dental filling compositions. In this case the connecting arms are limited to alkyl chains of eight carbons or less.
U.S. Pat. No. 4,952,241 discloses (meth)acrylic acid derivatives containing urethane groups prepared by the reaction of di-(meth)acrylic acid esters with diisocyanates, followed by subsequent reactions with polyols, and the use of these compounds in dental materials.
Dental impression materials that are reaction products formed from di- or tri-isocyanates with a combination of dihydroxy and unsaturated monohydroxy reagents are described in U.S. Pat. No. 4,182,829.
U.S. Pat. No. 4,691,045 discloses particular (meth)acrylate oligomers and their use in making unsaturated urethanes that can be used for curable coatings and adhesive compositions.
U.S. Pat. No. 3,825,518 discloses the use of urethane di(meth)acrylates as monomers for dental filling materials wherein the connecting arms for the (meth)acrylate to the urethane core is an alkylene group. Preferred materials have the alkylene group containing 2 to 10 carbon atoms.
The dental composites that have been made using the urethane (meth)acrylates described in the above references do not sufficiently meet the need for efficient and effective monomers for dental composite materials that combine reduced shrinkage with sufficiently low viscosity, high polymerization rate, and acceptable mechanical properties. We have found that the urethane (meth)acrylate monomers described herein, having relatively long, conformationally flexible arms, instead of short arms as previously taught, meet this need.